Third Phase:
When the Marian Reforms occur, the first of Rome's named and numbered Legions will appear in any of the regions you control that are listed below. These first Legions, of which there are 13, could also be called 'Consular Legions'. They were the paid army of Rome, and recruited when Rome was still a Republic. These can only be recruited and retrained in the specified areas. However, when the Marian Reforms occur, all pre-Marian Roman units will disappear from the ques and be replaced with various 'Auxilia' units. Auxilia, Light Auxilia, Archers and varius light Cavalry will be available everywhere to recruit. The 'House Rule is ONE EACH of these Legions. (I'll explain later what a Legion is)
Fourth Phase:
The Imperial Legions, of which there are 22, will appear in the same ten AOR Bases when you build an 'Urban Barracks'. At this point, all the Legions will be available IF you have a Royal Barracks in each region. The 'House Rule is ONE EACH of these Legions.
Fifth Phase:
The Praetorian Legion becomes available in Rome only once you've built the 'Castra Praetorium' in Rome. Praetorian Cavalry will also appear there, and the Praetorian Prefect. The 'House Rule' is TWO of these Legions.